An adverb expressing the wish for something to last for a long time (often used for marriages or long-term relationships).
ご結婚おめでとうございます。末永くお幸せに。
Congratulations on your marriage. Wishing you lasting happiness.
末永くお付き合いくださいと手紙に書いた。
I wrote in the letter, 'Please continue our relationship for many years to come.'
店主は末永く地域に愛される店にしたいと言った。
The shop owner said he wanted the store to be loved by the community for many years to come.